The Master Domain Model adds several levels of complexity over the Single Domain Model. Most importantly, the use of trusts, account distribution, and resource distribution. Trusts establish the linkage and hierarchy of the relationships between the domains.
Another important concept is the addition of two new terms: master domain and resource domain. A master Domain is the domain which contains all of the user's accounts, and in the Master Domain Model there is only one master domain. The master domain is the trusted domain. A resource domain is the domain that contains departmental or geographic servers, file and print servers, and other resources; it is the trusting domain. Under the Master Domain Model, there can be as many resource domains as necessary. This can help to reduce inter-LAN (or WAN) traffic. However, it should be noted that domains are logical groupings of machines, and there does not have to be any relationship bewteen domains and physical location.
Microsoft recommends against using the Master Domain Model for more than 15,000 users. However, due to performance considerations I would recommed a smaller number of about 7500 users.